The ras transforming genes of the Harvey and Kirsten strains of murine sarcoma virus and the homologous, evolutionarily conserved germline genes encode a set of related 21,000 dalton proteins. These p21 proteins can exert potent effects on cell proliferation and differentiation. We have produced and characterized monoclonal antibodies to the p21 encoded by Harvey murine sarcoma virus, and have demonstrated that these reagents can be used to study the expression of other viral and cellular forms of p21, both in rodents and in humans. We have cloned the ras gene of Harvey sarcoma virus into a protein expression plasmid vector which allows the synthesis of p21 in Escherichia coli. The bacterial-produced p21 retains its characteristic biochemical activities. The availability of monoclonal antibodies and producer bacterial strains should facilitate the further purification and biochemical characterization of p21.